Transparent plastic materials, i.e., high molecular weight organic polymers, have been widely used in a variety of optical and glazing applications. For example, transparent plastic materials are widely used in optical lenses, goggles, face shields, face plates for helmets, glazings used as windows in buildings, and glazings used as windshields or windows in buses, airplanes, and other transportation vehicles. However, transparent plastic materials tend to be soft and scratch or mar quite easily, which leads to reduced visibility and clarity through the transparent article.
Organosiloxane coatings have been used to help maintain visibility and clarity through the transparent plastic materials by providing a protective, abrasion resistant layer over the plastic material substrate. Protective, abrasion resistant organosiloxane coatings are typically applied to the transparent plastic material in liquid form and then are cured to form the protective coating. The polymeric components of the coating composition crosslink during the curing process to harden and form the protective, abrasion resistant coating.
Not all organosiloxane abrasion resistant coatings are compatible with the surface of the plastic material, and therefore, require a primer layer to provide the necessary adhesion for the coating to remain adhered to the plastic.
Certain transparent plastic materials, such as polycarbonate polymers, are formable. Formable materials are those in which the starting material is made pliable in some manner, and is then formed (shaped) and set into a desired, finished shape. Thermoforming is one type of forming process that may be used on formable materials. Thermoforming is a manufacturing process where a plastic sheet is heated to a pliable forming temperature, formed to a specific shape in a mold, and trimmed to create a usable product. More specifically, the sheet, or “film” when referring to thinner gauges and certain material types, is heated in an oven to a sufficiently high temperature that it can be stretched into or onto a mold and cooled to a finished shape. In many glazing and optical applications, it may be beneficial or even necessary to apply an abrasion resistant coating to the transparent plastic material prior to forming the plastic into the finished shape. However, not all such abrasion resistant coatings are compatible with the forming processes, such as thermoforming, because the coatings may crack or craze when the underlying transparent plastic substrate is subjected to changes in shape. In particular, abrasion resistance and formability in organosiloxane protective coatings generally share an inverse relationship. Abrasion resistance in the organosiloxane coating correlates to the hardness of the coating. A harder coating provides a more abrasion-resistant coating than a comparably less hard coating. However, harder organosiloxane coatings are less formable.